This invention relates to a sensor apparatus and method and in particular to a sensor apparatus and method for use in seismic surveys.
To conduct a geophysical survey of an underwater area in oil exploration or prospecting, it is known to use a multi-component sensor in conjunction with returning echoes produced by a seismic source. Such a sensor comprises an orthogonal array of three geophones. The seismic source is normally impulsive, produced by explosives or airguns; or continuous, produced by vibrator trucks. The source produces sudden pulses of short duration which are reflected and detected by the geophones.
In practice, a number of sensors are inserted in a cable arrangement. In Ocean Bottom Cable or in shallow water operations this cable is fed from a seagoing vessel and allowed to settle on the seabed.
Each geophone is an electromechanical device sensitive to vibrations along its axis. The signals from each geophone are amplified, fed into data processing equipment and arranged to produce a seismic reflection record. Measurement and analysis of seismic vibrations in each axis gives information regarding underlying strata composition.
For the seismic data to be of use, it is necessary to know the orientation of the geophones. Thus, the geophones are mounted on gimbals. The gimbals allow the geophones to rotate under gravity to align with the vertical. Once the cable settles, one of the three geophones is aligned in the vertical axis, and the others are aligned in the horizontal plane, one along the line of the sensor and one at right angles to the line of the sensor. That is, the sensor is mechanically gimballed.
Since on reaching the seabed a sensor can settle in any direction, alignment of the horizontal geophones relative to North is calculated from the seismic data at the time of processing the full seismic data set.
The quality of results provided by such a mechanically gimballed sensor is limited by the fact that vibrations act on each geophone via the coupling arm of the gimbals. Hence whilst any vibration does indeed cause a corresponding movement on the respective geophone, this vibration passes to the geophone via the coupling arm of the gimbals.
The effect of this indirect experience of vibrations on the geophones is frequency dependent. The effect ranges from attenuation of the signal at some frequencies, to oscillation of the geophones at the resonant frequency of the coupling arm.
In addition, vibrations in the horizontal axes can produce a swing of the gimbals which translates the movement of the vertical sensor to accelerations in its sensitive axis. The sensor cannot differentiate between actual seismic vibrations, and those caused by this cross-coupling.
This conventional apparatus and technique is costly, which restricts its use. In addition, vibration acting on each geophone via the coupling arm of the gimbals, and cross-coupling through the gimbals, give a high probability of error.
An object of the present invention is to provide a means to simplify this apparatus and method, and thus to reduce the cost and increase the accuracy of the survey by a significant factor.
According to a first aspect of the present invention there is provided a sensor apparatus for use in seismic surveys comprising a plurality of orientation units disposed in fixed positions relative to one another in an orthogonal array.
Preferably said seismic sensor apparatus comprises three orientation units.
In one embodiment a first orientation unit is disposed in a vertical axis (the xe2x80x98upxe2x80x99 axis) relative to the sensor apparatus. A second orientation unit may be disposed along the axis of the sensor apparatus in the horizontal plane (the xe2x80x98alongxe2x80x99 axis). In another embodiment a third orientation unit is disposed across the axis of the sensor apparatus in the horizontal plane (the xe2x80x98acrossxe2x80x99 axis).
Preferably each orientation unit comprises an accelerometer.
Typically each accelerometer is sensitive only to accelerations applied along the axis of the sensor in which it is disposed.
Preferably each orientation unit comprises means for receiving data representative of seismic movement of the earth""s surface.
In one embodiment the means for receiving data comprises the accelerometers.
Alternatively said means for receiving data comprises an analog geophone measuring vibration, coupled to an analog-to-digital converter. In this alternative a geophone is preferably fixed in the same axis as each accelerometer.
Preferably the sensor apparatus further comprises means to communicate seismic signals received to data processing software in a data processing unit remote from the sensor. In one embodiment some level of data processing is comprised in the sensor itself.
In a preferred form of the invention, the means to communicate data to the data processing unit comprises electronic or optical means.
In one embodiment the electronic or optical means is a transmission cable or fibre optic link.
Preferably the sensor apparatus is enclosed in a watertight container adapted to be lowered by cable to contact the seabed, the cable also providing a communication path for data.
From another aspect, the invention provides a method of conducting a seismic survey comprising the steps of:
positioning at least one sensor apparatus comprising three orthogonally arranged accelerometers in terrain of interest;
using the accelerometers of the apparatus to detect steady state acceleration;
using the sensor apparatus to collect seismic data produced by natural seismic events;
transferring data to a central location for analysis;
analysing the steady state acceleration data to determine the orientation of the apparatus; and
analysing the seismic data to measure the seismic vibrations.
In one embodiment the method further comprises the step of using accelerometers to collect seismic data.
Preferably the method further comprises inputting signals received by the accelerometers into data processing software. More preferably the method further comprises using data processing software to filter signals from the accelerometers to separate the steady state acceleration due to gravity and time varying signals due to seismic vibrations.
In another embodiment the method further comprises the step of calculating orientation of the system relative to the vertical. In one embodiment calculation of the orientation is implemented in real time. Alternatively, calculation of the orientation is made during processing of seismic data.
In one embodiment, the method further comprises the steps of transposing the seismic signal from the up axis accelerometer to give the seismic signal in the vertical axis; transposing the seismic signal from the along axis accelerometer to give the seismic signal in the horizontal along axis; and transposing the seismic signal from the across axis accelerometer to give the seismic signal in the horizontal across axis.
Preferably, each signal is referenced to the vertical and horizontal planes with the along line lying in the same vertical plane as the along axis of the sensor.
In one embodiment calculation of the transposition of the signal is implemented in real time. Alternatively, calculation of the transposition of the signal may be made during processing of seismic data.
Alternatively, the method comprises the step of using geophones to collect seismic data. Typically one geophone is fixed in the same axis as each accelerometer.
Preferably in this alternative the method further comprises the step of using each geophone to measure the seismic signal component in its axis. Typically, the method further comprises transposing seismic signals from each geophone to give the seismic signal in its required axis.
Typically, the method further comprises the step of using orthogonally disposed magnetic sensors to determine the orientation of the seismic sensor apparatus relative to North. More preferably, the seismic signals are transposed to a North and vertical reference frame. A more accurate calculation of vertical may be also made using alignment information from the magnetic sensors.